Previously known apparatus and methods provide phase-change ink to a multiple-orifice ink-jet print head, apply heat to melt the ink in a controlled manner, and selectively jet the melted ink toward an image-receiving medium, such as paper or some intermediate transfer medium, such as an image transfer drum to form a printed image. Phase-change ink is particularly advantageous because of its convenience, image quality, economy, and use of conventional print media.
In particular, U.S. Pat. No. 4,418,355 for an INK JET APPARATUS WITH PRELOADED DIAPHRAGM AND METHOD OF MAKING SAME describes a multiple-orifice ink-jet print head having an elongated serpentine-shaped heater element pressed against a heat-spreading ink reservoir wall plate for melting phase-change ink contained in the reservoir. A thermistor is inserted into a centrally located well in the ink reservoir wall plate to sense the ink reservoir temperature. The ink-jet print head reciprocates back and forth across a print medium while selectively jetting ink from piezoelectric transducer-driven jets to print an image.
Skilled workers know that an ink-jet print head ejects ink drops at a velocity that is determined by various parameters including the energy imparted to the ink by the piezoelectric transducer, the geometry of features in the head, and the ink viscosity. In particular, the viscosity of phase-change ink varies widely with temperature, a typical ink being solid at room temperature, rubbery near its 86 degree Celsius melting point, and a flowing liquid at its jetting temperature of about 130 degrees to about 140 degrees Celsius. Given a typical ink-jet head and a fixed amount of transducer energy, ink drop ejection velocity changes about two to about three percent per degree Celsius.
Because the ink-jet print head moves relative to the image-receiving medium while ejecting drops of ink, the landing points of the drops will vary in proportion to changes in drop ejection velocity. Therefore, to ensure acceptable drop landing accuracy, the phase-change ink temperature should be regulated and should be substantially the same for each jet of the multiple-orifice ink-jet print head. Ink temperature variations of greater than about three degrees Celsius can cause visible ink drop landing errors.
The voltage applied to the piezoelectric transducer of each jet can be "normalized," i.e., adjusted within a narrow range, to compensate for nonuniformities in jet construction and temperature, but such adjustment is often inadequate to compensate for the temperature nonuniformities within the print head. Furthermore, normalization is a factory adjustment that cannot dynamically adjust for changes in the thermal load on the ink-jet print head caused by environmental factors.
Factors causing temperature nonuniformity from jet to jet include nonuniform heat conduction losses, convection losses into the air, and radiation losses from the print head into adjacent objects. Convection losses are especially nonuniform in printers using a print head that reciprocates back and forth, thereby "fanning" the leading and trailing edges of the print head more than its central portions. Variations in the spacing between the ink-jet print head and the image-receiving medium cause temperature variations in the ink-jet print head because heat is more readily lost at closer spacings. Such spacing variations can occur, for example, if the ink-jet print head is mounted at a slight angle to the image-receiving medium. Other factors that dynamically change the thermal load on the print head include internal fans turning on and off, the actual printing process, access doors being opened and closed, and variations in the head to drum spacing.
Maintaining substantially the same ink temperature for each ink jet becomes more difficult as print heads become wider to accommodate additional ink-jet orifices. U.S. Pat. No. 5,087,930 for a DROP-ON-DEMAND INK JET PRINT HEAD, which is assigned to the assignee of this application, describes a 95-millimeter-wide, 96-orifice print head designed for ejecting phase-change inks. The ink-jet print head is attached to an ink reservoir that is mounted on a reciprocating carriage as described in U.S. Pat. No. 5,083,143 for ROTATIONAL ADJUSTMENT OF AN INK JET HEAD, which is assigned to the assignee of this application.
Referring to FIG. 1, a prior art ink-jet print head heater 10 was developed that generates more heat at the edges near its shorter side margins than at its central portion, in order to compensate for nonuniform convection losses near the shorter side margins of the 96-orifice print head. Heater 10 is a conventional flex circuit in which a heater foil 12 is formed from etched Inconel.RTM. (alloy 600) foil material laminated between a pair of Kapton.RTM. insulating layers. A heat-spreading copper foil layer is bonded to one of the Kapton.RTM. layers. Heater 10 is sized to match a major surface of the 96-orifice print head.
Heater foil 12 is electrically connected by a pair of contacts 14 to a temperature controller 16, which uses a single temperature sensor attached to the ink-jet print head. Temperature controller 16 applies a pulse-duration modulated voltage across contacts 14 in response to the temperature sensed by a thermistor 18. Heater foil 12 has a set of eleven adjacent heating areas 20 (shown generally as regions bounded by dashed lines) spaced across the X-dimension (width) of heater 10. Because electrical current flow is equal everywhere along heater foil 12, the watt-density in any area 20 is proportional to the electrical resistance of heater foil 12 in that area. The resistance of heater foil 12 is, therefore, made larger in heater areas 20 near contacts 14 than in heater areas 20 near thermistor 18. The watt-densities of heater areas 20 vary from about 2 to 2.5 watts per square centimeter near the center of heater 10 to about 3 to 3.25 watts per square centimeter at its left and right edges.
Thermistor 18 is embedded in a well in the 96-orifice print head. Access to thermistor 18 is gained through a cutout region 22 in heater 10. The location of thermistor 18 is not critical outside of the intended control area because the temperature sensed anywhere along the width of the 96-orifice print head is equalized elsewhere along the width of the print head by the zoned watt-density of heater 10. Because the phase-change ink is in intimate contact with the print head, equalizing the print head temperature also equalizes the ink temperature.
Another complication in print head design is that certain phase-change inks decompose if kept at an elevated temperature for extended periods of time. This decomposition places additional restriction on the thermal environment around the ink-jet print head, as well as additional demands on the ink-jet print head heater. For example, the reservoir and print head are in close proximity to allow ink to flow between them, but they are thermally isolated and have separate heaters and temperature sensors. Predetermined amounts of phase-change ink are melted and stored in the reservoir at a temperature slightly above the ink melting temperature, but significantly below the ink jetting temperature.
Co-pending U.S. patent application Ser. No. 07/965,812, filed Oct. 23, 1992, for a METHOD AND APPARATUS FOR PROVIDING PHASE CHANGE INK TO AN INK JET PRINTER, which is assigned to the assignee of this application, describes an ink-jet print head assembly having a premelt chamber, ink reservoir, and thermally isolated ink-jet print head. A printer using the ink-jet head assembly has start-up, idle, ready, and shutdown modes with each mode defining predetermined temperatures for the reservoir and print head. For example, in idle mode, the print head is kept at the same temperature as that of the reservoir, but when required to print, the print head temperature is rapidly elevated to bring the ink therein to its jetting temperature. The print head and its heater, temperature sensor, and temperature controller have a rapid thermal response time that reduces the time required to enter the ready mode and that acts to preserve the ink.
Phase-change ink-jet printers with reciprocating print heads produce high-quality images, but require a relatively long time to print each image. Print time can be shortened by increasing the number of jets printing the image. An ideal print head would span the full width of the image-receiving medium with ink-jet orifices spaced one picture element (hereafter "pixel") apart and would require only one scan of the print head relative to the print medium to print an image. It has been difficult, however, to produce such a print head. The nonuniform thermal loading on a media print head causes ink viscosity variation that adversely affects print quality. What is needed, therefore, is a substantially media-width, multiple-orifice, ink-jet print head having a heating system that heats the print head, and the phase-change ink contained therein, to a uniform temperature throughout the print head.